Amplifying circuits



B. J. THOMPSON ,122,851

AMPLIFYING CIRCUITS Filed Dec. 29, 1957 Inventor: BrowderJ-Tho p son, by 1/0 1 7 His Attorney. I

Patented July 5, 1938 UNITED STATES AMPLIFYING' CIRCUITS Browder J. Thompson, Basking Ridge, N. J., as-

signor to General Electric Company,

ration of New York Application December 29, 1937, Serial No. 182,267

4 Claims.

The present invention relates to an improved amplifying circuit especially useful in connection with control apparatus such as photosensitive devices which are adapted to produce very small potentials varying in accordance with a physical condition.

It is one object of the invention to provide a circuit of the type in question which can be operated satisfactorily from an alternating current power source.

Known types of photosensitive devices are characteristically of very high impedance and require to be used. with an amplifying circuit which has an input impedance of commensurately high value. However, it has been observed that as the input impedance of an amplifying circuit is increased, the relative importance of interelectrode capacitance also increases. With a circuit adapted to be energized by alternating current and having a sufiiciently high input impedance to assure proper cooperation with a conventional photosensitive device, the effect of interelectrode capacitance may be such as to prevent the normal functioning of the amplifier.

It is a more particular object of this invention as applied in connection with an amplifying circuit adapted for use with an alternating current source to provide means for effectively neutralizing the capacitance between the control grid and the remaining electrodes of the amplifier.

In accordance with a preferred embodiment of the invention this object is accomplished by suitably connecting a condenser between the control grid of the amplifier and a point of the alternating current source of supply whose voltage phase relation with respect to the connection point of the input impedance element cathode is opposite to that of the remaining electrodes. In this way the tendency of the control grid to accumulate charges as a result of its capacitive relation to the other electrodes is offset by the equal and opposite efiect of the neutralizing condenser.

The features of my invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure shows schematically a control circuit suitably embodying the invention.

Referring to the drawing, there is shown a photo-sensitive device (comprising a cathode I and an anode 2) connected to a vacuum tube amplifier which is designated as a whole by the numeral 4. This amplifier includes a cathode 5, an

anode 6, a control grid 1, a screen grid 8 and a space charge grid 9 whose nature and function will be more fully explained hereinafter. The output voltage of the amplifier, which is developed across a resistor It], may be impressed on the control grid of a suitable triggering or control device, exemplified in the present case by a thermionic discharge tube l2. For impressing a Y difference of potential between the various electrodes of the amplifier and associated devices there is provided a source of alternating current potential, illustrated as including a transformer having a secondary l3 and a primary M.

The photosensitive device is connected in the cathode-control-grid circuit of the amplifier 4 and is associated with an input resistance I 6. As previously noted, for purposes of cooperation with the high impedance photosensitive device this resistance should have an extremely high value, for example, on the order of 1,000 megohms.

As a result of the high impedance of the input circuit thus formed, the capacitance between the control grid and the anode and various other grids of the amplifier 4 may have a relatively important effect on the operation of the amplifier. These capacitances (indicated in dotted outline as condensers ll, l8 and I9) permit a substantial flow of alternating current through the resistor it. With a low value of the resistor Hi the resultant drop across it and the attendant swing of the control grid voltage may be negligible. However, with a resistance of the value required in the present connection the potential developed across the resistor from this cause may be sufficient to cause the control grid to swing positive at the same time as the grids 8 and 9 and the anode B and thus substantially to nullify the effect of variations in the current flowing through the photo tube and to prevent the normal functioning of the amplifier.

In order to overcome this difficulty, I provide a neutralizing condenser 22 connected between the control grid I and a point 23 of the alternating current source whose voltage phase relation with respect to the connection point 24 of the resistance !6 is opposite to that of the points to which the remaining electrodes are connected. By proper adjustment of this condenser it is possible to eliminate the blocking effect referred to and obtain. good performance of the amplifier. The requirement for neutralization is that the voltage between the points 23 and 24 multiplied by the capacitance of condenser 22 must equal the sum of 1) the voltage between the point 24 and the screen grid connection point multiplied by the capacitance between the control grid and screen grid plus (2) the voltage between the point 24 and the space charge grid connection point multiplied by the capacitance between the space-charge grid and control grid plus (3) similar products of voltage and capacitance for such other electrodes as may be involved. The simplest method of arriving at the proper value of the condenser 22 is to put in a small fixed condenser and to slide the tap along or to put in a small variable condenser with a fixed tap and to ad.- just the condenser until in either case satisfactory operation is achieved.

A further diificulty resulting from the use of the high resistance element 16 is the fact that any current however minute, which may flow from the cathode 5 to the control grid 1 during the half-cycle in which the latter is positive will produce a considerable voltage drop across the resistance I6. This drop will tend to charge the condenser 22, the interelectrode capacities I1, I8, and I9, and such stray capacities as may exist between the control grid and the associated elements of the circuit. During the portion of the alternating current cycle in which the control electrode is negative with respect to the cathode such charged capacities will oppose the action of the photosensitive device and render the control circuit unable to respond in the intended manner to the stimulus of a light beam impinging on the photosensitive element.

This difficulty is overcome by providing electrostatic means associated with the control grid 1 and the cathode 5 for preventing current flow between them. This. means may suitably comprise a space charge grid 9 which is adapted to be negatively charged whenever the control grid becomes positive with respect to the cathode, so that its efiect is to inhibit the movement of electrons from the cathode. As illustrated, the desired potential relationship is obtained by connecting the grid 9 to a point of the alternating current source whose voltage phase relation is opposite to that of the control grid when referred to the cathode as a reference point.

In order further to block the random electron current which tends to flow from the cathode during the period in which all the electrodes are at substantially zero potential, additional means are provided in the connection between the grid 9 and the transformer secondary 13 for definitely biasing the grid negatively with respect to the cathode 5 whenever the control element ceases to be negative with respect thereto. This means as illustrated comprises a condenser 20 in, parallel with a resistance 2!, these elements being so related in magnitude that any charge accumulated on the condenser will not be lost in less than a full cycle of the alternating current. Under these conditions current drawn by grid 9 during the period of normal conduction of the amplifier charges the condenser 20 and thus maintains a negative bias on the grid when a reversal of the alternating potential occurs.

The use of a space charge grid in this connection and apart from the use of a neutralizing condenser as claimed herein is fully described and claimed in my copending application S. N. 101,612, filed September 19, 1936 and assigned to the same assignee as the present invention.

The electrode 8 comprises a screen grid which may be interposed between the anode and the control grid if desired. This element does not constitute an essential part of my invention but if used will increase the effective amplification of the device and shield the control grid 1 from the effects of the anode field.

Although I have described my invention especially in connection with a photosensitive device it is by no means limited to such use and may be alternatively employed in connection with any electrical device, the electrical characteristics of which are variable automatically in. accordance with variations in a physical condition to which the device is subjected. Furthermore, while I have shown a particular embodiment of my invention, it will be understood that many modifications may be made by those skilled in the art without departure from the invention. I aim by the appended claims to cover all such modifications as fall within the true scope of the foregoing disclosure.

I claim:

1. In combination, an amplifier having a plurality of electrodes including at least a cathode, an anode and a control grid, an alternating current source having various points thereof connected to said electrodes for supplying desired potentials thereto, an input circuit including a high impedance element connected between the control grid and a point on said alternating current source, and a condenser connected between the control grid and a point of said source whose voltage phase relation with respect to the connection point of the impedance element is opposite to that of the points to which the electrodes of the amplifier other than the control grid are connected, the condenser being effective to neutralize the capacitance existing between the control grid and the said other electrodes.

2. In combination, an amplifier having a plurality of electrodes including a cathode, a control grid and an anode, an alternating current source connected to said electrodes ior impressing desired potentials thereon, an electrical device connected in the cathode-grid circuit of the amplifier and having its electrical characteristics variable automatically in accordance with variations in a physical condition to which the device is subjected, means including a high inipedance element connected between the control grid of the amplifier and a point on said alternating current source for causing variations in the electrical characteristics of said device to affect the potential of the control grid in a desired manner, and a condenser connected between said grid and a point of said source whose voltage phase relation with respect to the connection point of the impedance element is opposite to that of the points to which the electrodes of the amplifier, other than the control grid, are connected, the condenser being effective to neutralize the capacitance existing between the control grid and the remaining electrodes of the amplifier.

3. In combination, an amplifier having a plurality of electrodes including at least a cathode, an anode, a control grid, and a screen grid, an alternating current source for impressing desired potentials on said electrodes, a photosensitive device connected in the cathode-control-grid circuit of the amplifier, a resistance element of at least about 1,000 megohms connected between the control grid and a point of said alternating current source, and a condenser connected between the control grid and a point of the alternating current source whose voltage phase relation with respect to the connection point of the resistance element is opposite to that of the points to which the remaining electrodes,

other than the control grid are connected, the condenser being of suflicient capacity to neutralize the capacitance existing between the control grid and the said remaining electrodes.

4. In combination, an amplifier including at least an anode, a cathode, a control grid between the anode and cathode, and. a space charge grid between the cathode and control grid, an alternating current source for impressing desired potentials on said electrodes, a photo-sensitive device connected in the cathode-control-grid circuit of the amplifier, a high impedance element connected between the control grid and a point on said alternating current source associated with the cathode-control-grid circuit, means connecting the space charge grid to a point of said source whose voltage phase relation with respect to the cathode is opposite to that of the connection point of said impedance element, and a condenser connected between the control grid and a point of said source whose voltage phase relation with respect to the connection point of the impedance element is opposite to that of the space charge grid, the condenser being effective to neutralize 10 the capacity existing between the control grid and the space charge grid.

BROWDER J. THOMPSON. 

